The FAQ says to use "sudo $HOME/Library/Application\ Support/Steam/SteamApps/common/Shadowgrounds/shadowgrounds.app" to launch the app (after first running it once through Steam up to the crash to get the settings set up). When I do this, I get the exact same crash, on both 10.9 and 10.10, as just running it through Steam.

From what I can tell, "sudo open" doesn't actually do much of anything; the "open" command will still talk to the user's normal LaunchServices server and launch the app as the user, not as root. For example, if you "sudo open /Applications/TextEdit.app" then look at Activity Monitor or ps, it's owned by you, not by root. So, I tried "sudo $HOME/Library/Application\ Support/Steam/SteamApps/common/Shadowgrounds/shadowgrounds.app/Contents/MacOS/shadowgrounds" as well. That does actually launch the program as root, so if that were the problem, it would solve it. But I get the same results.

I thought the problem might be caused by the newer style of GPU-switching in modern Macs; on a few really old SDL programs, you have to use gfxCardStatus to force it to "Discrete Only" before launching the game or it crashes in a similar way. But that didn't help here.

The crash report doesn't seem too illuminating; my guess would be that an old version of libSDL or libSDL_video did something that's not thread-safe that happened to work up until 10.8, but 10.9 started dispatching the internals off to another thread, and trying to find out what that something is and patch it would be a lot harder than just getting a new version of the SDL libraries that works on 10.9. But I'll attach it anyway. And the syslogs, in case they're helpful. I've sent the same info in an email to support.

External Modification Summary:Calls made by other processes targeting this process:task_for_pid: 1thread_create: 0thread_set_state: 0Calls made by this process:task_for_pid: 0thread_create: 0thread_set_state: 0Calls made by all processes on this machine:task_for_pid: 252244thread_create: 0thread_set_state: 0